1. Field of the Invention
The present invention is generally related to semiconductor wafer processing equipment and, more particularly, to an improved apparatus and method for controlling plasma uniformity in a semiconductor wafer processing system.
2. Description of the Related Art
Traditionally, plasma-enhanced reactive ion etching systems contain an anode and cathode within a vacuum chamber. The cathode typically forms a pedestal for supporting a semiconductor wafer within the chamber and the anode is formed of the walls and/or top of the chamber. To process a wafer, a reactive gas is pumped into the vacuum chamber and the anode and cathode are driven by a single sinusoidal frequency (RF) source to excite the reactive gas into a plasma. The single frequency is typically 13.56 MHz, although frequencies from 100 kHz to 2.45 GHz are often used, with the occasional use of other frequencies. More specifically, a single frequency, sinusoidal RF signal is generally applied to the reactive gas within the chamber at a relatively high-power level, e.g., 3 kilowatts. The RF power excites the reactive gas, producing a plasma within the chamber proximate to the semiconductor wafer being processed. Such plasma-enhanced reactive ion processing has been used, for example, in etch and chemical vapor deposition processes.
The uniformity of the etch process that results from the forgoing etch chamber is poor. As such, an improved version of this etch chamber adds four magnetic coils about the outside of the chamber, i.e., one vertically oriented toroidal coil for each side of the chamber. These coils when driven with an AC signal magnetically control the plasma to facilitate a more uniform etch process. This form of chamber is generally known as a magnetically enhanced reactive ion etch (MERIE) chamber. One such MERIE chamber is manufactured by Applied Materials, inc. of Santa Clara Calif. as they model MxP.sup.+ chamber. This chamber is described in commonly assigned U.S. Pat. No. 5,215,619 issued Jun. 1, 1993 to Cheng et al. and U.S. Pat. No. 5,891,350 issued Apr. 6, 1999 to Shan et al., which are herein incorporated by reference.
FIG. 1 is a schematic diagram of a MERIE system 100 of the prior art. The system 100 includes a processing chamber 101. The chamber 101 comprises a set of side-walls 102, a floor 104 and a lid 106, defining an enclosed volume. A gas panel 110 supplies reactive gases (an etch chemistry) to the enclosed volume defined by the chamber 101. The system 100 further includes an RF power supply 122 and a matching circuit 120 that drives a pedestal assembly 108 such that an electric field is established between the pedestal assembly 108 and the chamber walls 102 and lid 106. A set of coils 124 are arranged about the sides 102 of the chamber 101 to facilitate magnetic control of the plasma 124.
A pedestal assembly 108 comprises a pedestal 114 centrally mounted within the chamber 101 to a cathode 112 and surrounded by a collar 118. The pedestal retains a workpiece 116 such as a semiconductor wafer which is to be processed in the chamber 101. The plasma reaction chamber 101 employs capacitively coupled RF power to generate and maintain a high density, low energy plasma 124. RF power, is coupled from the RF power supply 122 producing one or more RF frequencies through matching network 120. The lid 106 and walls 102 are grounded and serves as a ground reference (anode) for the RF power. With the configuration shown in FIG. 1, plasma density is controlled by the RF power provided by the power supply 122 via the matching circuit 120.
In semiconductor wafer processing, the cathode 112 is typically fabricated from a conductive material such as aluminum. The pedestal 114 is typically fabricated from a polymer such as polyimide or a ceramic material such as aluminum nitride or boron nitride. The workpiece 116 (i.e., a semiconductor wafer) is typically made of silicon. The electric field that couples to the plasma passes through both the workpiece and the pedestal. Since the cathode is made of a different materials than the workpiece, the different materials have different effects on the plasma. Consequently, there is an abrupt change of plasma parameters, and process uniformity, at the wafer edge 126. To improve process uniformity at the wafer edge, a collar 118, surrounds and partially overlaps the pedestal and pedestal 114. The collar 118 (also known as a process kit) is typically made of a material such as quartz.
Although magnetic enhancement provides a substantial improvement in etch uniformity, the existence of a magnetic field in the chamber causes a phenomenon known as E.times.B drift where the electrons in the plasma tend to accumulate on one side of the chamber. Such drift causes non-uniform etching. To combat E.times.B drift, the phase of the AC signal that drives each coil is rotated such that the B field is magnetically rotated. This lessens the E.times.B drift problem but does not eliminate it. Additionally, non-uniformity of ion energy and radical component density causes non-uniform etching of a wafer.
Therefore, there is a need in the art for an apparatus and method for improved control of the plasma uniformity as well as ion energy and radical component uniformity across the wafer surface to provide for more uniform and repeatable etching of wafers.